1. Field of the Invention
The present invention relates generally to a process for preparing a polymer composition for use as a coating or adhesive. More particularly, the invention relates to a process for preparing a low viscosity, high molecular weight non-aqueous dispersion polymer with improved shear strength for use as a coating or adhesive.
2. Description of the Prior Art
Over the past three decades, a significant body of knowledge has been developed on the emulsion polymerization of unsaturated molecules where the continuous phase is water. These emulsion polymers now constitute the base for water-borne coatings, adhesives, binders and paints. The benefits of water as the continuous medium are that it is non-flammable, nontoxic, free of odor and inexpensive. Nevertheless, water has the disadvantage of high heat of evaporation compared to organic solvents resulting in greater energy requirements to dry water-borne coatings. In addition, water-borne coatings are sensitive to irreversible damage by freezing, they cause grain rising when applied to wood and they cause rust when applied to unprotected ferrous metals. Furthermore, emulsion polymerization requires the presence of stabilizers, primarily surfactants which can adversely affect the end use performance of the polymer. The surfactants tend to make the resultant coating or polymeric film water sensitive compared to an identical coating or polymeric film from a solvent based coating.
While solvent-borne coatings and adhesives do not suffer these disadvantages, they also have certain limitations. In general, coating and adhesive compositions require a polymer of high molecular weight. However, the viscosity of such polymer solutions rises sharply with concentration and molecular weight of the dissolved polymer. For example, a 50% solids aqueous emulsion of a polymer, having a molecular weight of one million or more can have a viscosity of less than 100 centipoise, whereas the same polymer dissolved in a solvent would be a virtual gel with 50% dissolved solids. Consequently, in order to have a solvent-borne coating solution with low viscosity, the solids content of the solution must be around 30%. The disadvantage in this case is that the molecular weight of the dissolved polymer will be one or two orders of magnitude lower than that of the emulsion polymer. To compensate for the lower molecular weight coating, typically such polymers will contain functional groups which will crosslink with themselves or with additives when the coating is dried and/or baked. The disadvantage of crosslinking is that it can produce brittle, non-extensible films. The formulator of coatings is constantly faced with trading off of one property for another. If the coating must have some extensibility, for example, in the case of an exterior paint subject to wide temperature fluctuations, crosslinking must be limited and the tensile strength of the film will be lower.
It has been recognized that a non-aqueous dispersion polymer would have many of the advantages of both water-borne and solvent-borne coatings. These advantages include: (a) high solids coatings at low viscosities, (b) high molecular weight polymers without resorting to extensive crosslinking, and (c) elimination of water sensitivity such as grain raising and film plasticization by moisture.
A number of dispersion polymerizations of acrylic esters and methacrylic esters in organic continuous phases have been reported. The desired monomers, i.e., (meth) acrylic esters, are polymerized in a suitable organic continuous phase in the presence of a dispersant. The dispersant is generally a block or graft copolymer of two components, one soluble in and the other insoluble in the continuous phase.
These dispersion polymerizations, while improving the overall properties of the coating, have not been completely successful in achieving many of the advantages of both water-borne and solvent-borne coatings.